In the last decades much effort has been made in development of micropumps due to an increasing demand in different microfluidic applications like microanalytics, micro reaction technology, lab-on-a-chip applications, point-of-care diagnostics and drug-delivery devices. Other fields of interest include, but are not limited to, micro-dispensing of lubricants, fragrances, perfumes, scents, adhesives, nutrients, fertilizers and the like. Controlled release of beneficial agents over a prolonged period of time is one field of special interest.
Osmotic micropumps are designed to control displacement of fluids over prolonged periods of time and are therefore appropriate for many applications. A typical arrangement of an osmotic pump comprises at least one compartment containing an osmotic agent that is at least partially in direct contact with a semi-permeable membrane. The osmotic agent generates an osmotic potential and hence a pressure across the semi-permeable membrane and thereby imbibes solvent through the membrane. The solvent can be taken from another compartment or, e.g. in respect to implantable osmotic pumps, be imbibed from the surrounding media of the pump. The osmotic process generates a liquid phase by dissolution or dilution of the osmotic agent. The liquid phase can be used directly or be deployed as a driving fluid to displace a pumped fluid from another compartment. If necessary, the driving fluid and the pumped fluid can be separated by a piston, a flexible or elastic impermeable membrane or other movable means preventing contamination of the pumped fluid.
Osmotic pumps have many advantages over other micropumps such as low costs of manufacture, reliability, pressure capability and other aspects.
Commonly known osmotic pumps, however, can hardly be switched on and off instantly in an easy way or provide different flow rates between which one can easily switch. These aspects, however, are desired for many applications where e.g. a very low basal flow rate is desired with an additional initial or intermittently flushing at much higher flow rates. It is also often desired to have different basal flow rates available. In many drug delivery applications it is also desired, that a bolus dosage of the therapeutic substance can be delivered when needed in addition to the basal delivery rate.
Osmotic pumps are for instance described in WO 2005/107835 A1, U.S. Pat. No. 5,672,167, U.S. Pat. No. 4,505,702, U.S. Pat. No. 4,619,652 and the publication by Theeuwes and Yum, ANNALS OF BIOMEDICAL ENGINEERING, Vol. 4 No. 4, pp. 343-353, December 1976.
U.S. Pat. No. 3,604,417 describes an osmotic pump for long-term injection of a medicament. The osmotic pump includes a first part comprising a compartment filled with a concentrated solution. The compartment is delimited on one side by a moving piston and on another side by a membrane. The first part is inserted into a second part of the osmotic pump filled with a solvent to form a solvent chamber. The second part includes a moving piston to prevent formation of air pockets in the solvent chamber.
WO 2004/062714 A1 describes an automatic hydrogel-based extracorporal fluid conveyor. The fluid conveyor includes a swellable hydrogel.
WO 94/05354 A1 describes a fluid driven dispensing device with a piston driven by a fluid discharged from an osmotic engine.
An extracorporal osmotic pump is described in U.S. Pat. No. 4,193,398. The osmotic pump includes a first chamber containing an osmotic fluid and a second chamber containing water. A semipermeable membrane is arranged between the two chambers. The second chamber is formed by a polymeric bag.